1. Field of the Invention
The present invention relates to a discharge lamp ballast circuit for controlling lighting of a discharge lamp such as an HID (High Intensity Discharge) Lamp.
2. Description of Related Art
It is necessary for vehicle-mounted HID lamps, for example, to produce a breakdown by applying a short pulse voltage of about 20 kV to start a discharge.
As for a conventional discharge lamp ballast circuit, a built-in igniter generates a high short pulse voltage, and supplies it to the HID lamps.
The built-in igniter of the conventional discharge lamp ballast circuit has the following configuration.
Specifically, when the voltage across a capacitor connected in parallel with a spark gap exceeds the on-voltage of the spark gap, the spark gap is turned on.
Thus, a pulse current flows through the primary winding of an igniter transformer, which produces a short high voltage pulse of about 20 kV across the secondary winding, there by applying the short high voltage pulse to the electrodes at both ends of the HID lamps (see, Relevant Reference 1, for example).
Relevant Reference 1: Japanese patent application laid-open No. 2002-352989 (paragraphs [0014] and [0015], and FIG. 1)
With the foregoing configuration, the conventional discharge lamp ballast circuit can start the discharge of the HID lamps by applying the high voltage (short pulse voltage) of about 20 kV to the HID lamps. However, since it must apply the high voltage every time to start the HID lamps, it has a problem of wearing the electrodes of the HID lamps by sputtering, and hence shortening their life.
In addition, the igniter transformer of the igniter must have a large turn ratio for producing the high voltage of about 20 kV, and have a high dielectric withstand voltage between windings. This presents a problem of hindering reduction in size and cost.
Furthermore, to achieve the start of the HID lamps without fail at the first attempt, it is necessary to use a highly reliable spark gap, which prevents an increase in yields and reduction in cost of the component.
Incidentally, the conventional discharge lamp ballast circuit turns on the lamp at a low frequency of 400 Hz to circumvent the problem of acoustic resonance. Consequently, as a device connected in series with the lamp for generating the ignition voltage, it can use only a reactance that can transfer energy at a low frequency.